Various processes are known from the prior art for production of a metal layer on a substrate body. Many of these proposals deal with the way in which structured metal layers, for example in the form of antenna coils or flat coils, can be produced. The term “structured metallization” is to be understood in the text which follows as meaning all profiles of conductor runs which are conceivably possible. The antenna coils are used, for example, in what are known as contactless chip cards or RFID tags. Antennas of this type, which do not necessarily have to have a coil-like profile (for example dipole antennas), are also used in security tags in department stores. Furthermore, a structured metallization according to the above definition may be any conductor run layout which is required, for example, to connect a plurality of electronic components on a printed-circuit board.
A known process for producing a structured metal layer on a substrate body, as carried out in the production of an antenna coil for a contactless chip card, is to be described briefly below.
An aluminium foil with a thickness of 25 to 50 μm is laminated on to both sides of a substrate body which consists of a nonconductive material and has a thickness of approximately 30 to 50 μm. Initially, therefore, the structure is a laminate which has a metallization over the entire surface of the two main sides of the substrate body. To produce the structured metallization, an etching resist is applied to those sides of the aluminium layers which are remote from the main surfaces of the substrate body, and the resist is then exposed by photolithography. In the subsequent etching step, those regions of the aluminium layers covering the entire surface which are not required for a conductor run are etched away. After the etching resist which has remained on the structured metallization has been removed, the desired conductor structure is complete. The process described, which makes use of a subtractive principle, has the drawback that only low throughput rates can be achieved, a high consumption of chemicals is required and large parts of the raw materials used (aluminium layer) are wasted on account of the subtractive process. A process of this type is known, for example, from DE 196 29 269 A1.
An alternative process for the production of a structured metal layer consists in conductive pastes which are provided with conductive metal particles being applied in ready-structured form to a substrate body. The application may in this case be effected by means of a printing operation. Examples are given in WO 99/65002, DE 198 41 804 or WO 97 21118.
Conductive pastes have the drawback of having a high electrical resistance. Furthermore, they are generally very expensive, since the conductive particles usually consist of silver, and the proportion of silver in the conductive paste is over 90%. Moreover, on account of the high electrical resistance, it is necessary to bring about reinforcement by electrodeposition, so that a conductive layer, for example of copper, is formed on the conductive paste and is subsequently responsible for the actual electrical function. The conductive particles in the conductive paste consequently serve only to allow the growth of the metal layer by means of an electrodeposition operation. In addition to the high costs, the high thickness of >50 μm of the conductive paste is also worth noting, with the result that the thickness of the structure comprising substrate body and metal layer increases.
Another process is given in DE 198 10 809 C1. In this process, a metal powder layer is applied to the top side of a substrate film, and the top side of the substrate film, which has been covered with the metal powder, is then exposed by means of an imaging unit using electromagnetic radiation at the locations which correspond to the conductor tracks of the predetermined conductor structure. The metal powder is partially melted in the regions which have been exposed by the electromagnetic radiation, so that the melted metal powder is fixedly joined to the substrate film. Then, the substrate film is guided around a guide roller, so that the top side of the substrate film is oriented downwards, during which process the metal powder which is not joined to the substrate film is detached therefrom under the force of gravity, so that only the structured metal layer remains on the substrate film. The metal powder which falls off is collected and returned for reuse. It is unlikely that all the unmelted metal powder will drop off the top side under the force of gravity, and consequently further work is required.